imx8mq: Generate a single bootable flash.bin again

All i.MX8MQ boards have been converted to binman, which makes it
necessary to flash both flash.bin and u-boot.itb to get a bootable
system. Prior to the conversion, only flash.bin was needed.

Such new requirement breaks existing distro mechanisms to generate the
final binary because the extra u-boot.itb is now required.

Generate a final flash.bin that can be used again as a single bootable
binary to keep the original behavior.

After this change the SPL binary is called spl.bin, which is a more
descriptive name for its purpose, and can still be used standalone (for
example, for secure boot purposes).

Also update imx8mq_evk.rst to remove the u-boot.itb copy step.

This is a cherry-pick of 028abfd9 ("imx8mm-evk: Generate a single
bootable flash.bin again") adjusted to apply to i.MX8MQ.

Signed-off-by: Patrick Wildt <patrick@blueri.se>
Reviewed-by: Fabio Estevam <festevam@denx.de>